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The present invention relates generally to non-contact capacitive displacement measurement gages, and more specifically to non-contact capacitive displacement measurement gages capable of measuring displacements of target elements having unknown or uncontrollable impedance.
Non-contact capacitive displacement measurement gages are known that employ at least one capacitive probe to measure the displacement of a target element. In a conventional capacitive displacement measurement gage, a capacitance developed between a probe and a target element is converted by a signal processor into an output signal that is representative of the distance from the probe to the target. Specifically, the probe effectively forms one plate of a capacitor, and the target element effectively forms the other plate of the capacitor. To convert the capacitance between the probe and the target into an output signal representing the displacement of the target element, an electrical connection is normally required between the target and the probe and/or the signal processor. For example, this electrical connection may be made through a ground or xe2x80x9ccommonxe2x80x9d connection. Accordingly, the electrical connection may be made via a wire to ground, or by connecting the target and the probe and/or the signal processor to a common connection.
One drawback of the conventional capacitive displacement measurement gage is that it typically provides accurate displacement measurements only when there is a good electrical connection between the target and the probe and/or the signal processor, e.g., when the target element, the probe, and/or the signal processor are well-connected electrically to a common connection. For such a well-connected target, the relationship between the probe/target capacitance and the displacement of the target can be precisely determined. However, such good electrical connections often cannot be easily made between the target and the common connection, e.g., when the target is a semiconductor wafer disposed on a non-conductive surface. For such poorly-connected targets, it is difficult to determine a precise relationship between the probe/target capacitance and the target displacement because an impedance between the target element and the gage may be either unknown or poorly controlled.
One solution to the problem of making capacitive displacement measurements of poorly-connected targets is disclosed in U.S. Pat. No. 3,990,005 (the xe2x80x9c""005 patentxe2x80x9d) issued Nov. 2, 1976 entitled CAPACITIVE THICKNESS GAUGING FOR UNGROUNDED ELEMENTS. The ""005 patent discloses a capacitive dimension gage including two probes that are driven out-of-phase with each other and configured to remove the dependency on well-connected targets for achieving high accuracy displacement measurements. By providing drive currents having equal magnitude and opposite phase to the respective probes, the capacitive dimension gage prevents current from flowing through an impedance between the target element and the common connection, and therefore eliminates any adverse effect that this impedance would normally have on the operation of the gage. However, the capacitive dimension gage disclosed in the ""005 patent also has drawbacks because it requires the use of multiple probes, which can significantly increase the cost and reliability of the gage.
It would therefore be desirable to have a highly accurate non-contact capacitive displacement measurement gage. Such a capacitive displacement gage would provide high accuracy displacement measurements of both well-connected and poorly-connected targets. It would also be desirable to have a capacitive displacement measurement gage that has increased reliability and a reduced cost of manufacture.
In accordance with the present invention, a non-contact capacitive displacement measurement gage is disclosed that provides high accuracy displacement measurements of well-connected targets and poorly-connected targets. The presently disclosed capacitive displacement measurement gage employs at least one capacitive probe configured to drive substantially zero current into a target element, thereby nullifying any adverse effect an imperfect electrical connection between the target element and the gage may have on gage performance.
In one embodiment, the capacitive displacement measurement gage comprises at least one capacitive probe, at least one amplifier, and at least one signal generator. The capacitive probe includes a sensor electrode, a guard electrode, and a compensating electrode. The signal generator is configured to provide a predetermined voltage signal (1) directly to the sensor electrode, (2) to the guard electrode through a unity-gain amplifier, and (3) to the compensating electrode through a second amplifier having a predetermined transfer function. In the presently disclosed embodiment, the capacitive displacement gage is configurable to measure the displacement of at least one target element having an imperfect electrical connection to the gage, i.e., the electrical connection between the target element and the gage has an unknown or poorly controlled impedance. The second amplifier coupled between the signal generator and the compensating electrode is configured to assure that substantially zero current is driven into the target element by the probe during gage operation.
By providing a capacitive displacement measurement gage that employs a compensated capacitive probe, highly accurate displacement measurements of target elements can be made even if there is an unknown or poorly controlled impedance between the target element and the gage.
Other features, functions, and aspects of the invention will be evident from the Detailed Description of the Invention that follows.